A recently developed type of secondary or rehargeable electrical conversion device comprises: (1) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; (2) a cathodic reaction zone containing (a) a cathodic reactant comprising sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversibly reactive with said anodic reactant; (b) a solid elecrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with said anodic and cathodic reaction zones; and (c) electrode means within said cathodic reaction zone for transporting electrons to and from the vicinity of said cation-permeable barrier. As used herein the term "reactant" is intended to mean both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the surfaces of the electrode by reaction of the cathodic reactant with electrons conducted through the electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the electrode in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte and the alkali metal cations can pass out of the solid electrolyte into the liquid electrolyte and combine to form alkali metal polysulfide near the solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur. As can be readily appreciated the production of large amounts of sulfur near the surface of the cation-permeable membrane has a limiting effect on rechargeability. This is the case since sulfur is nonconductive and when it covers surfaces of the electrode, charge transfer is inhibited and the charging process is greatly hindered or terminated. Thus, in order to improve the rechargeability of a cell of this type it is necessary not only to supply polysulfide to the surface of the electrode in the vicinity of the cation-permeable membrane, but also to remove sulfur therefrom.
Numerous suggestions have been made for improving the mass transportation of cathodic reactants so as to improve charge and discharge efficiency as well as to increase the ampere hour capacity of the battery or cell.
U.S. Pat. No. 3,811,943 and U.S. Pat. application Ser. No. 545,048 filed Jan. 29, 1975 both disclose energy conversion device designs which allow or promote improved mass transportation of reactants and reaction products to and from the vicinity of the solid electrolyte and the porous electrode during both discharge and charge. In the device disclosed in the patent an ionically conductive solid electrolyte is located between a first reactant in one container and a second reactant in another container. An electrode for one of the reactants comprises a layer of porous electronically conductive material having one surface in contact with one side of the ionically conductive solid electrolyte and the other surface in contact with a structurally integral electronically conductive member permeable to mass flow of its reactant and electrically connected to the external circuit. An open volume exists between the structurally integral conductive member and the container wall to promote free flow and mixing of the reactant. Reactants also flow readily through the conductive member into the layer of porous electronically conductive material. The conductive member distributes electrons to the porous, conductive material which in turn transfers electrons to or from the reactants.
The improvement disclosed in the patent application comprises designing the cathodic reaction zone of the device such that there are a plurality of channels and/or spaces within said zone which are free of porous conductive electrodes and which are thus adapted to allow free flow of the molten cathodic reactants during operation of the device. This flow results from free convection within the channels and/or spaces, and from wicking of cathodic reactants within the conductive porous material.
The prior art designs disclosed and claimed in the aforementioned U.S. patent and in Ser. No. 545,048 are effective in promoting distribution of reactants during both discharge and charge. However, even with these improved designs it is difficult to recharge the cells or batteries at high rates.
U.S. Pat. application Ser. No. 567,464 filed Apr. 14, 1975 and Ser. Nos. 605,941 and 605,942 now U.S. Pat. No. 3,951,689 all teach ways of improving the ampere-hour capacity as well as charge and discharge efficiency of such batteries or cells. However, each relies on vapor transport of sulfur to accomplish its purpose. This either complicates cell design or requires additional external heating or cooling.
U.S. Pat. application Ser. No. 653,865 filed concurrently herewith and entitled "Secondary Battery or Cell with Polysulfide Wettable Electrode" discloses an improved cell with increased charge efficiency resulting from the use of a polysulfide wettable electrode. Such cells, while showing significant increases in charge efficiency, do demonstrate appreciable electrode polarization on discharge and, thus decreased discharge efficiency.
It has been found that a battery or cell showing both increased efficiency on charge and increased efficiency on discharge can be achieved by combining the improvement of the above-mentioned concurrently filed application with still further improvements of this invention.